Titanium oxide particles are added as a filler having a high refractive index to coating solutions and resin compositions. Such coating solutions and resins are used for various applications and, in particular, they are preferably used as coating solutions for forming films for high-refractive-index optical base materials such as eyeglass lenses.
Coating films for optical lenses need to have transparency and hence are preferably formed of titanium oxide fine particles having a small particle diameter. However, titanium oxide fine particles having a small particle diameter have a high activity and hence the following problem is caused: the weather resistance or light resistance of a film or a base material having a film is degraded and the film becomes separated, or a film tends to suffer from yellowing or bluing.
To suppress such degradation of a film and to enhance the weather resistance of a film, Patent Literature 1 describes a coating film containing inorganic oxide fine particles in which the surfaces of core particles containing titanium oxide are covered with a coating layer composed of a composite oxide of silicon oxide and zirconium oxide and/or aluminum oxide.
Patent Literature 2 describes titanium oxide ultrafine particles in which the surfaces of titanium oxide particles are covered with amorphous zirconium oxide and a coating solution containing the fine particles.